Motor improvement for self-rotating displays

ABSTRACT

A self-rotating display device includes and outer light transmissive container ( 402 ) containing a light transmissive fluid ( 406 ) and an body ( 404 ) containing an electric motor ( 421 ) for rotating the body with respect to the outer container. The body also carries an amount of the fluid ( 430   a ) contacting the fluid in the outer container through a pressure equalizing gap ( 431 ) in the body which forms a fluid pathway between the inner cavity of the body and the inner chamber of the outer container. The fluid pathway forms self-regulating pressure relief structure which accommodates slight pressure variations in the fluid due to climactic conditions for example. A specialized reduced footprint fluid-immersible electric motor having separate field and compass magnets, which do not rotate relative to each other, helps eliminate magnetic cogging. The device can be manufactured according to a method which eliminates the necessity of a fill hole in the body.

PRIOR APPLICATION

This is a divisional of U.S. patent application Ser. No. 15/539,615,filed 2017 Jun. 23 which is a 371 of International Application SerialNo. PCT/US2015/000453, filed 2015 Dec. 24 which claims the benefit ofU.S. Provisional Patent Application Ser. No. 62/096,983 filed 2014 Dec.26 and U.S. Provisional Patent Application Ser. No. 62/152,714 filed2015 Apr. 24, all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to self-powered display devices, and moreparticularly, to enclosed, fluid immersed, light-powered, electric motordriven self-rotating devices.

BACKGROUND

Self-moving displays are often used as toys, decorative conversationpieces or advertising media. Such devices are disclosed in my U.S. Pat.Nos. 6,275,127; 6,853,283; 6,937,125; and U.S. Pat. Publication No.2005/0102869; all of which are incorporated herein by reference.

These devices can have a sealed outer container having lighttransmissive walls containing a light transmissive liquid whichbuoyantly supports an inner body which appears to magically rotate onits own with respect to the outer container, or in what appears to be asolid block of clear glass or plastic. The rotation can be driven by anelectric motor hidden within the body. The motor can be powered by abattery or in a longer-term manner by light radiation impacting onphotovoltaic cells hidden within the body.

One problem can occur due to atmospheric pressure and humiditydifferences occurring naturally at various localities throughout theworld over time. For example, during a winter storm in Denver, Colo. thepressure and humidity may be far less than during the summer in Rio DeJaneiro. Due primarily to manufacturing and safety concerns, the outercontainer of the device is often made of a relatively non-hermeticmaterial such clear acrylonitrile butadiene styrene (ABS). Therefore,changes in atmospheric pressure and humidity can seep through the wallsof the outer container and change the water content and vapor pressureof the inner light transmissive liquid. These changes, coupled withchanges in temperature can cause the total liquid volume inside thecontainer to become bigger or smaller than the volume available for thefluid. If the liquid volume is bigger, it can cause an overpressurepotentially damaging the device. An underpressure can cause theformation of an unsightly bubble at the top of the display, potentiallyruining the magical appearance of the device. These changes can alsolead to changes in the buoyancy of the inner body when it contains anamount of gas due to a Cartesian diver effect.

Another potential problem is that these devices can rely on an internalcompass magnet aligned with an ambient magnetic field such as theearth's magnetic field to act as a source of counter-torque for theirinternal motors. In such devices there has been a possibility of amagnetic interaction between the compass magnet and field magnets thatare used to generate a relative torque as they interact with coils ofwire carrying currents of the on-board electric motor. FIG. 1 showsgraphically how this magnetic interaction can cause a speed variation inthe motor. It is clear that as the driving current to the motor isreduced, the motor will stop at a much lower drive current than it wouldwithout this interaction negatively impacting operation in low-lightconditions.

This problem can be reduced by designing the field magnet structure tominimize the magnetic interaction and also by mounting the compassmagnet far from the field magnets. However, practically speaking, thisreduction can be expensive and can detrimentally increase the size ofthe powering mechanism in a device having limited space.

Therefore there is a need for a self-rotating device which addressessome or all of the above identified inadequacies.

SUMMARY

The principal and secondary objects of the invention are to provide animproved fluid suspended, self-rotating device. These and other objectsare achieved by a pressure equalizing fluid pathway through the wall ofa fluid suspended, self-rotating body.

In some embodiments there is provided a specialized electric motorhaving separate compass and field magnets that do not rotate withrespect to one another.

In some embodiments the self rotating body is bouyantly supported withinthe container by two different density immiscible fluids.

In some embodiments, there is provided a self-rotating device whichcomprises: an outer container having an internal cavity shaped anddimensioned to contain an amount of a first fluid and a self-poweredhollow rotating body immersed in said first fluid; wherein said bodycomprises: an outer wall; an upper chamber; a lower chamber; a fluidimpermeable, light transmissive bulkhead separating said upper and lowerchambers; a conduit passing through said outer wall and defining aninner passageway in fluid communication between said cavity and saidupper chamber; and, and, a second amount of said fluid located in saidupper chamber, and and amount of air in said upper chamber.

In some embodiments said fluid comprises two different density liquids,wherein said liquids are selected to buoyantly support said body withinsaid outer container.

In some embodiments said fluid consists of a single homogeneous liquidand wherein said device comprises a mechanical support rotativelysuspending said body in said fluid.

In some embodiments, there is provided a rotating container whichcomprises: a container housing; an electric motor comprising: a compassmagnetic for aligning to an ambient magnetic field; at least one fieldmagnet apart from said compass magnet; a mechanical linkage fixing anorientation of said field magnet to an orientation of said compassmagnet; at least one coil mechanically fixed to said container; whereinsaid at least one coil is located to interact with a magnetic fieldgenerated by said at least one field magnet; and, a current supply forsupplying commutated current to said at least one coil.

In some embodiments, there is provided a self-rotating device comprises:a light transmissive outer container having an internal cavity shapedand dimensioned to contain an amount of a fluid and a self-poweredhollow rotating body immersed in said fluid; wherein said outercontainer is sealed; wherein said body comprises: an outer wallsubstantially impermeable to said fluid; an inner chamber; at least oneconduit passing through said outer wall and defining a fluid passagewayin fluid communication between said cavity and said inner chamber; and,a portion of said fluid located in said inner chamber, and an amount ofa gas in said inner chamber.

In some embodiments, said amount of fluid comprises a lower densitycomponent fluid and a higher density component fluid, wherein said lowerdensity component fluid is substantially immiscible with said higherdensity component fluid.

In some embodiments each of said component fluids are denser than air.

In some embodiments said amount of fluid comprises a first lower densityliquid and a second higher density liquid, wherein said first liquid issubstantially immiscible with said second liquid.

In some embodiments said conduit has a first aperture through said outersurface of said body located above an interface between said first andsecond liquids, when said device is at equilibrium in a gravity field.

In some embodiments said conduit has a first aperture through said outersurface of said body located above an interface between said first andsecond liquids, when said device is at equilibrium in a gravity field.

In some embodiments said body further comprises a top shell and a bottomshell joined along a seam by a bond.

In some embodiments said conduit comprises a gap between said top shelland said bottom shell along said seam.

In some embodiments said gap is formed by an absence of said bond.

In some embodiments said body is substantially spherical and whereinsaid cooperative mating surfaces occur at an equator.

In some embodiments said conduit remains permanently immersed in liquid.

In some embodiments, there is provided a self-rotating device comprises:an outer container having an internal cavity shaped and dimensioned tocontain an amount of a first fluid and a self-powered hollow rotatingbody immersed in said first fluid; wherein said body comprises: an outerwall; an upper chamber; a lower chamber; a fluid impermeable, lighttransmissive bulkhead separating said upper and lower chambers; at leastone conduit passing through said outer wall and defining an innerpassageway in fluid communication between said cavity and said upperchamber; and, a second amount of said fluid located in said upperchamber, and an amount of air in said upper chamber.

In some embodiments said fluid comprises two different density liquids,wherein said liquids are selected to buoyantly support said body withinsaid outer container.

In some embodiments said fluid consists of a single homogeneous liquidand wherein said device comprises a mechanical support rotativelysuspending said body in said fluid.

In some embodiments, there is provided a rotating body comprises: a bodyhousing; an electric motor comprising: a compass magnetic for aligningto an ambient magnetic field; at least one field magnet apart from saidcompass magnet; a mechanical linkage fixing an orientation of said fieldmagnet to an orientation of said compass magnet; at least one coilmechanically fixed to said body; wherein said at least one coil islocated to interact with a magnetic field generated by said at least onefield magnet; and, a current supply for supplying commutated current tosaid at least one coil.

In some embodiments, there is provided a method for manufacturing aself-rotating device comprises: selecting a bondable pair of upper andlower shells in an unjoined state; wherein said lower shell contains amotor component; wherein said lower shell has an smaller unfilled volumethan said upper shell; inverting said upper shell; filling said uppershell with a first amount of a liquid; bonding said lower shell to saidupper shell while in the inverted state to form a joined body; revertingsaid joined body to its upright orientation; and, placing said joinedbody into a container including a second amount of said liquid.

The original text of the original claims is incorporated herein byreference as describing features in some embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph shows how the speed of rotation can vary with rotationangle in prior art devices.

FIG. 2 is a diagrammatic cross-sectional side view representation of alight driven, motor containing, rotating body immersed in a lighttransmissive fluid contained in a light transmissive outer container.

FIG. 3 is a diagrammatic cross-sectional side view representation of amotor-containing body showing the drive construction and the pressurerelieving upper chamber according to an exemplary embodiment of theinvention.

FIG. 4 is a diagrammatic top partial transparent view of the body ofFIG. 3.

FIG. 5 an electrical circuit diagram for the electrical components ofthe body of FIG. 2.

FIG. 6 is a diagrammatic cross-sectional side view representation of amotor-containing body showing the drive construction and the pressurerelieving upper chamber according to an alternate exemplary embodimentof the invention.

FIG. 7 is a diagrammatic cross-sectional top view of the body of FIG. 6showing the pressure relieving conduit through the body wall.

FIG. 8 is a diagrammatic cross-sectional top view of a body having aplurality of pressure relieving conduits through the body wall.

FIG. 9 is a flow chart diagram showing the primary steps ofmanufacturing a fluid containing body.

FIG. 10 is a diagrammatic cross-sectional side view representation ofthe inverted, unjoined shells in the manufacturing process of FIG. 9.

FIG. 11 is a diagrammatic cross-sectional side view representation ofthe inverted joined shells in the manufacturing process of FIG. 9.

FIG. 12 is a diagrammatic cross-sectional side view representation ofthe reverted joined shells immersed in fluid in a contain in themanufacturing process of FIG. 9.

FIG. 13 is a diagrammatic cross-sectional side view representation of alight driven, motor containing, rotating body immersed in a singlehomogeneous light-transmissive fluid contained in a light-transmissiveouter container.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring now to the drawing there is shown in FIG. 2 a self-rotatingdevice 1 having a substantially stationary, sealed outer container 2having light transmissive walls 3 surrounding an inner cavity 5containing an amount of light transmissive fluid 6, and an axiallysymmetrically shaped body 4 such as a sphere or ball is immersed in thefluid and allowed to rotate about an axis 7 with respect to the outercontainer. The body has a light-transmissive wall 9 allowing ambientlight rays L to pass through the outer container wall 3, fluid 6, andbody wall 9 to provide power to a solar cell 15 supplying current to anelectric motor 14 inside the body. The axially symmetric shape of thebody allows it to rotate with a minimum amount of drag from contactingthe surrounding liquids. The earth's magnetic field 10 provides ananchor to a compass magnet 18 within the body providing a source ofcounter-torque to the motor. The fluid can comprise two immiscibleliquids, a less dense liquid 6 a, and a more dense liquid 6 b, separatedat an interface 8 as disclosed in French, U.S. Pat. Publication No.2005/0102869 incorporated herein by reference.

As shown in FIG. 3 the body 4 in this embodiment can include asubstantially hemispherical top shell 11 and a substantiallyhemispherical bottom shell 12 joined at an equatorial seam 13 to form aself-rotating, substantially spherical ball.

The body 4 can have a pressure relief structure which helps equalizepressures inside and outside the body. The body 4 includes an internalupper chamber 27 and an internal lower chamber 26 separated by aninternal light transmissive disk-shaped septum 25. The lower chamber canbe empty or filled with one or more fluids such as air, inert liquids orboth. A conduit 31 can be formed penetrating the body wall from an outeraperture 32 exposed to the outer container cavity 16, to an inneraperture 33 exposed to the body's upper chamber 27. The conduit thusdefines a fluid passageway 34 extending between the cavity 16 and theupper chamber 27. The passageway is in fluid communication between theliquid 6 a in the container cavity 16 and a portion of liquid 30contained within the body's upper chamber.

The inner end 35 of the conduit 31 surrounding the inner aperture 33terminates a distance or gap 36 above the septum 25. The volume of theportion of liquid 30 in the inner chamber 27 is selected to create adepth 37 above the level of the inner aperture and leave a volume ofless dense fluid such as air 38 located in the upper region of the innerchamber when the device is at equilibrium in a gravity field. Further,the volume should be sufficient to provide for a significant depth 37 tokeep the inner aperture 33 immersed during incidental movement such asshaking or tilting during transport. This arrangement allows for liquidwithin the conduit 31 to flow into and out of the conduit based on therespective pressures of the liquids 6 a,6 b in the inner cavity 16 andthe inner chamber 27. The volume of air 38, as a gas, is compressibleand can thus act to dampen the forces of differential pressures.

Those skilled in the art will readily appreciate the maximization ofparameters such temperatures and pressures which would tend to drivedown the level of the portion of liquid 30 within the body. Care shouldtherefore be taken to select a depth during nominal conditions whichprevents the level from lowering to an extent which exposes the inneraperture.

The conduit 31 is preferably made relatively narrow, having across-sectional area of between about 1.0 and 3.0 square millimeters tohelp prevent bubbles from passing through the conduit when the body 4 istemporarily inverted or tilted during transport or other movement. Thebody 4 is normally in the orientation shown in FIG. 3 due to a ringshaped ballast weight 22 near the lower part of the body. Usually thepressure differential between the upper chamber 27 and the outercontainer cavity 16 occurs very gradually. Thus a conduit having asmaller cross-sectional area is often adequate. Indeed, a conduit formedby what may be termed a leak is often adequate to allow pressureequalization. Further, by selecting a narrower conduit, surface tensionforces can temporarily prevent liquid remaining in the conduit fromescaping the conduit when the inner aperture 33 is briefly exposed toair. This can help prevent the passage of air through the conduit duringoccasional temporary disruption of the orientation of the body when thedevice is temporarily tilted. Air passage through the conduit would leadto unsightly bubbles in the liquid in the container cavity 16 outsidethe body 4.

When two immiscible liquids 6 a,6 b are used to form the amount of fluid6, the greater density liquid 6 b can be a humectant whereas the lesserdensity liquid 6 a can be selected to have a matching index ofrefraction to the humectant as described in French supra. Manyhumectants such as propylene glycol can be damaging to the internaldrive components of a motorized device. Therefore, in such situationswhere the internal drive mechanisms such as the motor and solar cellsare exposed to the fluid, it is preferred that the lesser density liquid6 a be the liquid that exclusively enters and exits the inner chamber27. Thus it is preferable to locate the conduit 31 outer aperture 32above the interface 8 between the two liquids when the device is atequilibrium.

Referring now to FIGS. 3 and 4 the device further includes a verticalshaft 20 connected to a compass magnet 41, and an upper iron disk 45 anda lower iron disk 47 spaced apart a fixed distance by a spacer 49 andperpendicular to the shaft. The shaft 20 is rotatively supported on thebottom by a hard rounded end 23 resting in a cup jewel bearing 24. A cupbearing holder 21 retains the cup jewel bearing and also locates andretains the ballast ring 22. A solar cell 42 a is shown mounted on aprinted circuit board 43. Brackets 17 mount the printed circuit board tothe bottom hemispherical shell 12.

FIG. 4 shows a top view of the structure of FIG. 3, with its major partsshown in transparency for clarity. It shall be noted that the angularorientation of the rotating parts of the motor are different betweenFIGS. 3 and 4. Three uniformly angularly spaced apart solar cells, 42a,42 b,42 c are shown mounted on the top of the printed circuit board 43and three uniformly angularly spaced apart bobbins wound with wire 48a,48 b,48 c are shown mounted on the bottom of the printed circuitboard. Four uniformly angularly spaced apart disk shaped magnets 50 a,50b,50 c,50 d are also shown mounted on the lower iron disk 47, two ofwhich, 50 a and 20 b are shown in FIG. 3. The spacer 49 passes through ahole 28 in the printed circuit board and the shaft 20 is in the centerof the spacer 49.

As the printed circuit board 43 rotates with respect to the iron disks45,47, each phototransistor 44 a,44 b,44 c is shaded by the upper irondisk 45 until it passes under one of the apertures 46 a,46 b. In FIG. 4,the phototransistor 44 a is shown passing under aperture 46 a, andphototransistors 44 b and 44 c are shaded by the top iron disk 45. Whileunder the aperture, phototransistor 44 a is exposed to light anddelivers current to its bobbin 48 a.

FIG. 5 shows the electronic circuit on the printed circuit board 43.Light falling on any of the phototransistors 44 a,44 b,44 c will createa current that is amplified by its respective transistor 51 a,51 b,51 cto drive current through coil its respective coil 48 a,48 b,48 c. Diodes54 a, 54 b,54 c protect the transistors in case any reverse voltage isgenerated if the relative rotation of the coils 48 a,48 b,48 c andmagnets 50 a,50 b,50 c,50 d is somehow forced to happen in reverse.Solar cells 42 a,42 b,42 c provide voltage to drive the circuit.

In the relative orientation of the printed circuit board 43 to the irondisks 45,47 shown in FIG. 3, the coil 48 a will be receiving currentbecause phototransistor 44 a is illuminated and this current will createa relative torque between the coil 48 a and the magnets 50 a and 50 b.The shaft 20 is held from rotating by the interaction of the compassmagnet 41 with an ambient horizontal magnetic field 10 such as theearth's magnetic field. The net result will be that the coil 48 a, theprinted circuit board 43 and the body 4 will feel a torque and start torotate if the body is in a low friction environment such as describedabove. Continued rotation will eventually cause phototransistor 44 a tobe shaded and expose another phototransistor 44 b or 44 c to be exposedthrough aperture 46 b and this will cause continued rotation.

It is important to note that since there is no relative rotation betweenthe magnets 50 a,50 b,50 c,50 d and the compass magnet 41, then therewill be no magnetic drag. This means the distance D between the compassmagnet and the field magnets can be much smaller than in prior motors ofthis type, thereby allowing room for other structures such as thepressure equalization structure described above. This is advantageousbecause any changes in the relative internal volume of the outercontainer 2 and the total volume of the inner cavity 16, including thebody 4, and the liquids 6 a and 6 b, will not result in excessivepressures and possible bursting of the outer container 2, because excesspressure will cause liquid 6 a to flow into the upper chamber 27 andslightly compress the air 38 above the portion of fluid 30 in thechamber.

The dimensioning of all the above components can be easily determinedknowing the coefficients of expansion, dimensions, and environmentalextremes expected. Net expansion of the outer container and its contentscan be caused by temperature changes and by water vapor passing throughthe material of outer container 2 such as plastic, as might be caused ina very humid environment.

Referring now to FIGS. 6 and 7, there is shown an alternate embodimentof a pressure relief structure implemented on a fluid immersed,self-rotating display device 101. In this embodiment the pressureequalizing conduit 131 can be efficiently formed by creating a gapbetween the two joined shells 111,112.

The device 101 includes an inner body 104 buoyantly suspended in a fluid106 contained within the internal cavity 116 of a translucent, sealedouter container, not shown, but similar to the container 2 shown in FIG.2. The fluid can include a first amount of lesser density liquid 106 asuch as a paraffinic liquid located within the upper part of the cavityand a second greater density liquid 106 b such as a humectant liquid,within the lower part of the cavity. The liquids 106 a, 106 b can beimmiscible, meeting along an interface 108, preferably made to have thesame index of refraction, and be adjusted in density and volume to causethe body 104 to float buoyantly near the center, vertically, of theouter container.

The body 104 includes a top substantially hemispherical shell 111 and abottom substantially hemispherical shell 112 bonded along a seam 113 toform a sphere or ball. This shape of the body is angularly symmetric sothat in can rotate with a minimum amount of drag from contacting thesurrounding liquids 106 a,106 b. The body has an internal chamber 127. Aconduit 131 penetrates the body wall 109 from an outer aperture 132exposed to the suspending fluid 106 to an inner aperture 133 exposed tothe inner chamber 127. The conduit thus defines a fluid passagewayextending between the suspending fluid and the inner chamber. Thus, thepassageway is in fluid communication between the suspending liquids 106a,106 b and a portion of liquid 130 contained within the inner chamber.

The amount of liquid 130 in the inner chamber 127 is selected to createa depth 137 above the level of the inner aperture 133 and leave a volumeof lesser density fluid such as air 138 located in the upper portion ofthe inner chamber. This arrangement allows for liquid within the conduitto flow into and out of the conduit based on the respective pressures ofthe suspending liquids 106 a,106 b and the amount of liquid 130 in theinner chamber 127.

The conduit 131 is dimensioned to be relatively narrow, to help preventbubbles from passing through when the body 104 is temporarily inverteddue to shaking. The conduit can be formed by an absence of adhesive usedto join the two shells together along their mutual seam 113. An angularzone Z along the seam of between about 5 and 20 degrees is selected forthe absence of adhesive. For a body having an outer diameter of about100 millimeters, an angular zone of about 10 degrees forms a conduitabout 25 millimeters wide and about 0.001 inch thick between the shells.

The body 104 is normally in the orientation shown because there is aring-shaped ballast weight 122 near its lower part. The amount of liquid130 in the inner chamber 127 can also provide ballast and damping to theother components contained within the inner chamber and immersed withinthe liquid.

FIG. 6 further shows alternately configured drive components for a lightenergized electric motor powered device. These components operatesimilarly to those described in the previous embodiment. A verticalshaft 120 is connected to a compass magnet 141, a top iron disk 145, aspacer 149, and a bottom iron disk 147 upon which is mounted fouruniformly angularly spaced apart disk magnets, two of which, 150 a and150 b are shown in FIG. 6. The shaft 120 is supported on the bottom by ahard rounded ball end 123 resting in a cup jewel bearing 124. A cupbearing holder 121 retains the cup jewel bearing and also locates andretains the ballast ring 122. A top bearing 126 rotatively engages thetop of the shaft. A protective cup structure 129 protects the compassmagnet from sloshing liquid in the event the body is overturned duringtransport. As with the previous embodiment light L falling on anunshaded phototransistor 144 will allow current to flow from the solarcells 142 through the appropriate coil 148 to generate a magnetic fieldwhich forces it away from disk magnet 150 b.

Referring now to FIG. 8, there is shown an alternate embodiment of ahollow rotating body 204 having a plurality of conduits 231 passingthrough the body wall 209 allowing fluid communication between an innerchamber 227 and the fluid 206 outside the body. Thus, in the event oneof the conduits becomes blocked or failed to form adequately duringmanufacture, the other conduits provide redundancy.

Referring now to FIGS. 9-12 there is shown a manufacturing method 301for assembling a self-rotating, fluid-immersed display device. Ingeneral the method involves the steps of: selecting the upper and lowershells of the rotating body in an unjoined state 302 where the lowershell contains the primary motor components so that it has an smallerunfilled volume than the upper shell; inverting the upper shell 303;filling the upper shell with an amount of lesser density liquid 304;bonding the lower shell to the upper shell while in the inverted state305 to form the joined body; reverting the joined body to its uprightorientation 306; and placing the joined body into a container includinglesser density liquid 307.

FIG. 10 shows the upper shell 411 and the lower shell 412 of thesubstantially spherical body 404 in an unjoined state. The upper shell411 can be inverted and a volume of lower density liquid 430 a pouredin. The level 437 of the liquid need not rise above the rim which is theexposed ring-shaped surface of the upper shell. Layers of adhesive 414can be deposited along the exposed, ring-shaped surfaces of one or bothof the shells 411,412 intended to contact one another. An absence ofadhesive is left along a corresponding angular zones 415,416 of bothshells.

Next, as shown in FIG. 11, while the adhesive 414 remains uncured, thelower shell 412 containing the motor components 421 attached thereto,also in an inverted orientation, can be mated to the liquid filled,inverted upper shell 411. Care must be taken to properly align the twoangular zones where adhesive is missing. The adhesive is allowed tocure. Where there is an absence of adhesive between the two shells alongthe mated angular zones, a gap 431 is formed which will act as the fluidconduit described above.

As shown in FIG. 12, the spherical body 404 in its joined hemispheresstate can be reverted to its upright orientation where the upper shell411 is on top of the lower shell 412. The volume of the liquid 430 apoured into the inverted upper shell 411 is selected so that when thejoined body is returned to its non-inverted state the liquid level 437will rise to a predetermined level which is a distance 432 above thelevel of the conduit 431. This rise in level corresponds to the volumeof the motor components 421 attached to the lower shell which becomeimmersed in the liquid. The body can then be buoyantly supported by anamount of fluid 406 including a greater density liquid 406 b and alesser density liquid 406 a as in a prior embodiment to rotate about itsvertical axis 99.

In this method the pouring of the liquid into the upper shell can beaccomplished much more rapidly than injecting the same volume of liquidthrough a hole in the joined shells. Further, the necessity of anunsightly fill-hole is eliminated.

An advantage of the present embodiment is that use of such a low profilemotor can benefit in the design of a ball-in-cube type structure byallowing space within the ball to house the pressure relief structure.

As shown in FIG. 13, a self-rotating display device 501 can include arotating body 504 having and internal light driven motor 514 one of thepressure relief structures shown in the prior embodiments, which isimmersed in a fluid consisting of a single homogeneous liquid 506contained within a sealed container 502 having light-transmissive walls503. The body may rest against the bottom surface 555 of the containercavity 505 and the bottom wall 556 of the container can be thickenedvertically in order to locate the body at a more pleasing mediallocation within the container.

EXAMPLE

A outer substantially cubic hollow container made of transparentacrylonitrile butadiene styrene (ABS) having a wall thickness of about 5millimeters and sides measuring about 15 centimeters square looselycarries a hollow spherical body of transparent ABS having a wallthickness of about 3 millimeters and a diameter of about 10 centimeters.The body is buoyantly supported inside the container by two immiscible,different density liquids. The first, higher density liquid is a mixtureof about 81% by volume propylene glycol and 19% by volume water. Thesecond, lower density liquid is dodecane. The body is formed by twohemispherical shells bonded along an equator by an amount of adhesive.An angular gap in the adhesive of about 10 degrees forms a conduit forthe flow of liquid between the inside and outside of the body.

Testing showed that the device tolerated exposure to temperatures up to50 degrees C., whereas devices without the gap burst at about 40 degreesC.

While the exemplary embodiments of the invention have been described,modifications can be made and other embodiments may be devised withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

What is claimed is:
 1. A self-rotating device comprises: a body; anelectric motor carried on said body and driving rotation of said body;said electric motor comprises: a compass magnetic for aligning to anambient magnetic field; at least one field magnet apart from saidcompass magnet; a mechanical linkage fixing an orientation of said atleast one field magnet to an orientation of said compass magnet; atleast one coil mechanically fixed to said body; wherein said at leastone coil is located to interact with a magnetic field generated by saidat least one field magnet; and, a current supply for supplyingcommutated current to said at least one coil.
 2. The device of claim 1,wherein said at least one coil comprises three uniformly spaced apartcoils.
 3. The device of claim 1, wherein said at least one field magnetcomprises four uniformly spaced apart field magnets.
 4. The device ofclaim 1, wherein said current supply comprises: at least one solar cell;at least one phototransistor wired in series between said solar cell andsaid at least one coil.
 5. The device of claim 4, wherein said devicefurther comprises: an upper light shading disk located between said atleast one phototransistor and a light source; said upper light shadingdisk comprising at least one aperture for intermittently allowing saidat least one phototransistor to be exposed to said light source during arevolution of said body with respect to said at least one field magnet.6. The device of claim 5, wherein said device further comprises: said atleast one phototransistor comprises three phototransistors angularlyevenly spaced apart from one another; said at least one aperturecomprises two apertures angularly evenly spaced apart from one another;wherein said two apertures are shaped and dimensioned to allow at leastone of said three phototransistors to be exposed to light at any giventime during a revolution of said body with respect to said at least onefield magnet.
 7. The device of claim 6, wherein said device furthercomprises a lower light shading disk axially spaced apart from saidupper light shading disk; wherein said upper light shading disk and saidlower light shading disk axially straddle said at least one field magnetand at least one coil.
 8. The device of claim 7, wherein said devicefurther comprises: said at least one field magnet comprises four fieldmagnets angularly evenly spaced apart from one another; and, said atleast one coil comprises three coils angularly evenly spaced apart fromone another.
 9. The device of claim 8, wherein said device furthercomprises: said four field magnets being fixed with respect to said twoapertures; and, said three coils being fixed with respect to said threephototransistors.
 10. The device of claim 9, wherein said upper lightshading disk is made from a first material comprising iron; and, saidlower light shading disk is made from a second material comprising iron.11. The device of claim 10, wherein said first material and said secondmaterial are substantially the same.
 12. The device of claim 1, whereinsaid device further comprises: a light transmissive outer containerhaving an internal cavity shaped and dimensioned to contain an amount ofa fluid and a self-powered hollow rotating body immersed in said fluid;wherein said outer container is sealed; wherein said body comprises: anouter wall substantially impermeable to said fluid; an inner chamber; atleast one conduit passing through said outer wall and defining a fluidpassageway in fluid communication between said cavity and said innerchamber; and, a portion of said fluid located in said inner chamber, andan amount of a gas in said inner chamber.
 13. The device of claim 12,wherein said amount of fluid comprises a first lower density liquid anda second higher density liquid, wherein said first liquid issubstantially immiscible with said second liquid.
 14. The device ofclaim 13, wherein said conduit has a first aperture through said outersurface of said body located above an interface between said first andsecond liquids, when said device is at equilibrium in a gravity field.15. The device of claim 14, wherein said motor is immersed in said firstlower density liquid.